The term phytosterol refers to a group of compounds, which are naturally occurring in plants. In the recent years, there has been a growing interest in these compounds due to their wide range of applications such as food and cosmetic additives as well as active component in various pharmaceutical formulations. An area, which is gaining special interest in the past few years and which additionally increases the demand for phytosterols and their derivatives, is the area of so called functional foods where the active substance has cholesterol-lowering effect upon scheduled use. It has been proven that plant sterols and their derivatives reduce cholesterol levels in human blood.
The compounds within phytosterol group comprise one of the two branches of a larger steroid group. The other branch of steroid group is comprised of compounds found in humans and animals with typical example being cholesterol. Steroids are terpenoid lipids characterized by carbon skeleton which is comprised typically of four fused rings (most often in 6-6-6-5 fashion, where numbers 6 and 5 denote the number of carbon atoms in each ring). Most often, each ring within the fused ring structure is denoted by a capital letter, thus the four fused rings structure is often written as A-B-C-D, where D corresponds to cyclopentane-ring. There are hundreds of steroid members known and characterized, where the main difference is in their functional groups. When hydroxyl (—OH) functionality is attached to the ring skeleton (usually to the 3rd carbon atom in the A-ring), the steroids are referred to as sterols. All phytosterols are based on the cycloartenol-type sterol, whereas all animal steroids are based on the lanosterol-type sterol.
Generally, the phytosterols are concentrated and isolated during vegetable oil processing where both edible and non-edible oils can be considered as potential candidates for phytosterol source. Thus, all crops utilized for vegetable oil production can be used as phytosterol sources, where typical examples include but not limited to oils obtained from soybean, canola, corn, cottonseed, palm, etc. The phytosterols in vegetable oils are present in free form and/or as steryl esters (SE, esters formed between corresponding phytosterol and fatty acid), where the total phytosterol content is typically in the range of up to one percent by weight. During the vegetable oil refining, residual streams enriched in phytosterols can be obtained and used as sources for subsequent sterol isolation.
An alternative source for phytosterols is Tall Oil (TO) a non-edible vegetable oil, which is a by-product product available at the pulp and paper mills. The tall oil is comprised of lipophilic extractives of wood. During wood cooking (typically via Kraft-type process) these extractives are solubilised into the cooking liquor through alkali assisted hydrolysis. Thus obtained cooking liquor is concentrated further in series of evaporation steps. At certain concentration, the solubilised lipophilic components naturally separate from the remaining aqueous phase and are skimmed-off from the liquor. The obtained stream is often referred to as tall oil soap or just soap. The tall oil soap typically is acidulated at the mill sites to obtain an oil phase, tall oil, and a brine aqueous solution. The obtained TO or more often referred to as crude tall oil (CTO), is typically exported to centralized tall oil refineries for further upgrading.
The CTO is comprised of an acidic fraction and a neutral fraction. The acidic fraction is further sub-divided into free fatty acids (FFA, 35-60 wt. %) and resin acids (RA, 15-55 wt. %), where the components of both fractions are characterized by the presence of carboxylic acid functionality (—COOH). The neutral fraction (5-35 wt. %) on the other hand, is comprised of a large number of compounds such as alcohols, aldehydes, ketones, hydrocarbons, etc. The common feature for these compounds is that they are not prone to reaction with alkali and hence often referred to as unsaponifiables. The phytosterol-type components dominate within the TO neutral fraction and beta-Sitosterol is the principal component of the neutral fraction. Further, the phytosterols are present in tall oil mainly as free sterols because of the hydrolysis conditions during the wood cooking. Nevertheless, some steryl esters can be also found in TO, where the amount of steryl esters is mainly dependent on the tall oil origin, pre-treatment and storage conditions.
Tall oil upgrading typically involves one or more vacuum distillation steps, where the objective is to obtain the two principal component fractions, namely those of free fatty acids and resin acids. Upon their removal, a high boiling-point fraction remains as bottom stream which is enriched in phytosterols and is usually referred to as Tall Oil Pitch (TOP) or just pitch. The obtained TOP is typically used as low cost energy source at various industrial sites. In light of phytosterol isolation, the tall oil pitch is preferred source over the initial tall oil due to the reduced volumes to be processed. Typical sterols present in TO and respectively in TOP include beta-Sitosterol, Stigmasterol, Campesterol, though their saturated counterpart's beta-Sitostanol, Stigmastanol, Campestanol, respectively can be found also in minor quantities.
Over the years many processes have been developed for phytosterol isolation from by-product streams within Pulp & Paper industry. Depending on the stream enriched in phytosterols, different strategies have been adopted.
Although the preferred source stream for sterol isolation is TOP, some process disclosures on previous art describing sterol isolation from tall oil soap and TO streams have to be mentioned, since the ground principles within these disclosures are applied often at certain process stage within the tall oil pitch upgrading.
Soap stream is attractive source for phytosterol isolation because the major fraction i.e. acidic fraction is in the salt form, typically sodium salts whereas the neutral fraction containing the sterols is not affected by the alkali. Further, all phytosterols are present into the soap stream are already as free phytosterols. The U.S. Pat. No. 3,965,085 and U.S. Pat. No. 3,803,114 describe similar strategies for the isolation of neutral fraction from soap streams available at the Pulp & Paper mills. The common feature for these processes is the use of hydrocarbon-based solvent to extract the neutral fraction whereas the acidic fraction of the soap remains in the aqueous phase. In order to facilitate the separation and prevent the formation of stable emulsions auxiliary solvent is introduced into the system, ketone or low-molecular weight alcohol as described in U.S. Pat. No. 3,965,085 and U.S. Pat. No. 3,803,114 respectively. Although demonstrated on commercial scale, the processes have many disadvantages which can be summarized as: (i) large volumes to be processed; (ii) need for large volumes of solvents used within the extraction step; (iii) need of high quality soap (essentially free of black liquor) otherwise extensive problems with phase separation between soap (aqueous) and hydrocarbon based solvent phases; (iv) need for solvent recovery loops (minimum two solvents are utilized); (v) need for further phytosterol isolation from the other neutral components extracted from the soap; etc.
Phytosterol isolation has been demonstrated on TO streams as well. The phytosterols within TO stream are mainly as free sterols but certain amounts of steryl esters are also found. The amount of steryl esters depends on the TO origin, pre-treatment conditions as well as storage conditions and duration.
U.S. Pat. No. 2,280,843 discloses a process for the preparation of sterol concentrate from TO. The TO stream is dissolved in suitable solvent and the obtained common stream is passed through a bed of appropriate sorbent which retains the neutral fraction of the TO. The retained neutrals can be liberated by passing through the adsorbent bed different type of solvent in which neutrals are readily soluble. Eluted neutral fraction is concentrated by means of solvent evaporation to render neutral oil from where the sterols can be isolated in an additional step.
WO 2004/080942 discloses a process for the CTO fractionation into FAAE's, RA's and sterol streams. In this process, the TO is modified first by means of selective esterification of FFA with alkyl alcohol to the corresponding FAAE's, followed by sterol esterification with boric acid to obtain the corresponding sterol borate esters. The modified tall oil is fractionated via vacuum distillation to obtain sterol borate ester concentrate, which is used to isolate the free sterols via hydrolysis of borate esters.
CA 2349780 discloses a process for the sterol isolation from TO stream. In this process, the initial CTO is distilled to remove the light oil fraction, containing the FFA's and RA's, and the residue containing the sterols. The residue is further fractionated into a distillate containing concentrated free sterols and a high boiling point residue. The sterols are isolated from the sterol concentrated distillate via crystallization in suitable solvents.
The process disclosures on phytosterol isolation from tall oil have many disadvantages similar to the case of tall oil soap: (i) large volumes to be processed are involved; (ii) the use of number of solvents is involved; (iii) heavy distillation conditions are involved, in certain cases a number of distillation steps are needed; (iv) in most of the cases the steryl esters or in more particular the sterols within these esters are not recovered which may substantially decrease the total sterol yield.
From commercial point of view, the tall oil pitch is particularly interesting since the phytosterols and their derivatives are most concentrated. A number of processes have been disclosed on the phytosterol isolation from TOP. However, the state of phytosterols in the TOP differs significantly from the one in tall oil soap and tall oil. In the TOP phytosterols are present typically as steryl esters and only minor amounts of free sterols. The presence of phytosterols in the form of steryl esters to large extent predetermines the possible processing schemes for phytosterol isolation from TOP.
WO 99/42471 discloses a process where the sterols are isolated from TOP by complete saponification of the TOP to obtain the FFA's and RA's in their alkali salt forms and liberate the bound sterols. The obtained soap phase is acidulated to obtain TO enriched in sterols, which is further distilled to obtain a light volatile distillate fraction comprised of FFA's and RA's and a residue fraction. The residue fraction is subsequently distilled to obtain sterol concentrate, which in turn is used for the sterol isolation by applying suitable solvents.
U.S. Pat. No. 2,715,638 and U.S. Pat. No. 3,691,211 disclose similar processes for sterol isolation from TOP. The acidic fraction of the TOP is neutralized with water-alcoholic alkali solution. The separation of the soap phase and the oil phase in some cases is facilitated by addition of auxiliary non-polar solvent as described in the U.S. Pat. No. 3,691,211. The soap phase is discarded, whereas the oil phase containing bound sterols is subjected to hydrolysis conditions where the steryl esters are hydrolyzed to free sterols and corresponding alkali salts of the FFA's. Upon cooling the sterols crystallize and can be separated.
The U.S. Pat. No. 2,715,639 discloses a process for sterol isolation from TOP via direct saponification of the TOP water-alcohol alkali solution. The obtained soap phase is diluted with large amount of water and allowed to cool-down. Upon cooling the sterols crystallize and can be separated.
The WO 00/64921 discloses a process for sterol isolation and purification from TOP. The TOP is first completely saponified to hydrolyze all steryl esters to free sterols and FFA's. The neutral fraction of TOP is extracted by art similar to the one described for extraction of tall oil soap. The obtained neutral fraction is further upgraded to phytosterols of high quality through preparation of sterol-metal aducts, which are separated and subsequently hydrolyzed to obtain free sterols.
There are number of disadvantages concerning the process disclosures on phytosterol isolation from tall oil pitch which can be summarized as: (i) the use of alkali treatment step typically complete saponification of TOP; (ii) generation of an additional soap streams that typically need to be acidulated to produce an oil stream; (iii) use of heavy distillation conditions to further concentrate the sterols and (iv) the use of large volumes of solvent mixtures to concentrate and/or isolate the sterols.
Although, the residual streams which have potential for phytosterol isolation can differ significantly in their bulk matrix composition, the main criteria for selection of isolation procedure is the sterol state i.e. whether the major sterol fraction is comprised of free sterols or steryl esters—bound sterols. There is an obvious need for universal procedure for phytosterol recovery which (i) allows the isolation of phytosterol fraction of high quality and high yield (ii) is independent of the particular characteristics of the source stream (free sterols and/or steryl esters) and (iii) that eliminates most and preferably all of the disadvantages of existing practices for phytosterol recovery listed earlier.
In the following we describe a process for isolation of phytosterol fraction from tall oil pitch in high quality and yield. Furthermore, we believe that because of its universal philosophy, the process can be adapted to any type of residual stream concentrated in phytosterols.